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In symmetrical grids, single-input single-output (SISO) analysis/design tools are applicable for the analysis and design of grid-tied inverter systems as their average αβ- or dq-frame models are to a high extent decoupled (in the high-frequency range). In this way, designing the control system could be done in a straightforward manner. However, this is not the case under asymmetrical grids. In this condition, the interaction between different phases makes the average model of the grid-tied inverter a multi-input multioutput (MIMO) system, and therefore, complicated to analyze and design. To cope with this challenge, this article deals with deriving the MIMO control system considering the impact of the asymmetrical grid. Then, the MIMO system turns into two decoupled SISO systems. Thus, a simple yet accurate model is developed which intuitively demonstrates the effect of the grid asymmetry on creating coupling between current control loops. In this way, the stability analysis could be carried out irrespective of the MIMO system difficulties. The experimental results validate the accuracy of the proposed approach in asymmetrical grid conditions. [ABSTRACT FROM AUTHOR]

In this article, a new inverter unit is proposed. It is a compact switched-capacitor-based inverter, which is combined of four switches, two diodes, and four capacitors. Since this inverter unit is a mid-point-clamped inverter, it can mitigate the capacitive leakage current in many applications. One of the main applications of the proposed inverter is in grid-tied photovoltaic systems, where the common-mode current brings many challenges. Other than the mentioned feature, the proposed inverter includes the following advantages: 1) spontaneous balancing of the voltage across the capacitors without needing any control system or auxiliary circuits; 2) capability of full reactive power transfer; 3) low voltage stress on the capacitors compared to common three-level inverter topologies (NPC, T-type-NPC, FC, etc.); and 4) symmetric switching sequence, which causes a continuous input current. The feasibility and versatility of the proposed inverter unit is validated by experimental tests, which are conducted by using a lab-scale prototype. [ABSTRACT FROM AUTHOR]

Bipolar dc grid systems are fast gaining attraction for renewable energy source integration, because of their merits of higher reliability, efficiency, and robustness as compared to the unipolar dc grids. However, the progress in multiport converters, resulting into lower cost and more compact design for bipolar microgrid systems, is fairly slow. Therefore, this article proposes a novel family of five nonisolated multiport dc–dc converter topologies with bipolar symmetric outputs. The performance and key operational features of the proposed converters under varying input voltages, duty cycles and loads are numerically verified and experimentally on an in-house test setup to prove the concept of the proposed converters. In the experimental validation, the operation of the converter under simultaneous and arbitrary individual power transfer from two input ports is tested. Furthermore, the easy integration of the proposed converters with a multilevel inverter to achieve high-quality ac voltages is demonstrated. As compared to the few existing counterparts, the proposed converters have a competitive edge in terms of higher number of input ports and voltage gains. Alongside the possibility of arbitrary independent power flow from the input ports, inherently symmetrical outputs require a simple balance control for asymmetrical members of the family. [ABSTRACT FROM AUTHOR]

In an era where high-frequency flying capacitor (FC) multilevel inverters (MLIs) are increasingly gaining attention in energy conversion systems that push the boundaries of power density, the need for a compact, fast, and accurate FC voltage monitoring system is also increasing. In this article, we designed and developed a new FC measurement system, based on precise sampling of the inverter switching node voltage, through a bidirectional clamping circuit. The deviation of FC voltages from their nominal values is extracted by solving a set of linear equations. With a single sensor per phase and no isolation requirements, as opposed to dozens of sensors in traditional FC monitoring, our approach results in significantly lower cost, complexity, and circuit size. Detailed device-level simulations in LTspice and system-scale simulations in MATLAB validate the accuracy and speed of the proposed measurement system and the balancing strategy in steady-state, abrupt load change, and imbalance conditions. Experiments carried out in a three-phase gallium nitride five-level inverter prototype reveal a gain in precision and bandwidth that is more than 30 times that of conventional methods, at a fraction of their cost and footprint. The recorded performance renders the developed sensor an ideal solution for fast MLIs based on wide-bandgap technology. [ABSTRACT FROM AUTHOR]

This article presents a transformerless high step-down dc–dc converter based on a stacked active bridge (SAB) configuration. The SAB converter consists of series-stacked capacitively coupled inverter modules and parallel-connected rectifier modules. The nominal step-down conversion ratio is determined by the number of the inverter modules, while the output current capability scales with the number of the paralleled rectifier modules. SAB operating principles, including phase-shift control and soft switching, achieved through series inductors, are similar to transformer-isolated dual active bridge converters. It is found that the best size versus loss tradeoff is achieved by integration of the series inductors on a custom core. Furthermore, it is shown that how soft switching contributes to natural voltage sharing among the series-stacked inverter modules. The approach is verified by experimental results on a 400–48 V, 3-kW SAB prototype using GaN devices and featuring 400- ${\mathrm{W/}}{\text{in}}^{3}$ power density. A flat efficiency curve is obtained with 99% peak efficiency, 97.5% full-load efficiency, and 98% efficiency at 20% load. [ABSTRACT FROM AUTHOR]

A charging strategy that transitions from constant current (CC) mode to constant voltage (CV) mode is typical for lithium batteries. This presents design challenges for inductive power transfer (IPT) chargers to achieve high efficiency. In this article, a multiband frequency tracking control is proposed to match CC and CV modes and to mitigate misalignment. As in prior work, two load-independent frequencies fCC and fCV are used for respective CC and CV outputs. Unity power factor can be achieved when fully aligned. For misalignment cases, two narrow frequency bands around fCC are selected for control to deliver target output currents and limit reactive power. The duty ratio of the inverter is controlled to set target output voltages. A single-stage IPT system with a series LCC topology is analyzed. The proposed control has advantages of narrow operating frequency ranges, high misalignment tolerance, direct design procedures, and high efficiency in both CC and CV modes. The method is verified on a 4.4 kW prototype with a 150 mm air gap. Experimental results show that the target CC and CV charging profile can be achieved with up to 120 mm of lateral misalignment. When fully aligned, the maximum system conversion efficiency reaches 97.5%. It stays above 95.5% and 93.3% over the entire CC and CV mode ranges. [ABSTRACT FROM AUTHOR]

To overcome the challenges of large reactive energy transmission loss and high current stress, which ocurr during the operation of auxiliary commutated circuit (ACC) of novel parallel resonant dc-link inverter (PRDCLI), an improved modulation strategy for reactive energy transmission loss is herein proposed based on the discontinuous pulsewidth modulation (DPWM), which adopts sawtooth carrier wave with alternating positive and negative slopes and shunt dead zone. Compared with the original modulation strategy, based on minimizing the ACC operation frequency, the proposed modulation strategy is able to realize all switches soft-switching, and avoid the superposition of resonant current and load current during the operation of ACC, thus, effectively reducing the reactive energy transmission loss and the current stress of auxiliary switches of the ACC. Under the proposed modulation strategy, the equivalent circuits in different operation modes are divided, and the operation principle, voltage/current stress, loss distribution, and optimal parameter design method of novel PRDCLI are researched. Eventually, the proposed modulation strategy is verified by experiments on a 5-kW/40-kHz prototype made by SiC mosfet. [ABSTRACT FROM AUTHOR]

Common-mode voltage (CMV) is one of the major concerns in modern motor drive systems using pulsewidth modulation (PWM) controls. To alleviate the CMV issue, a series of different PWM methods have been proposed. However, due to the physical limits of the three-phase structure, none of those reduced-CMV PWMs (RCMV-PWMs) can fully eliminate the CMV. As the motor phases have been extended to even numbers, such as six phases, opportunities to eliminate the CMV by a proper modulation scheme emerge. This article proposes a new modulation method to fully eliminate or reduce the CMV for the full modulation span of a dual-three-phase motor drive system, thereby helping to shrink the CM filter. Mathematical models are built indicating its effectiveness of eliminating the CMV in the linearity range of the modulation index and reducing the CMV in the overmodulation range. Simulation results along with experiments implemented on field-programmable gate array controlled dual inverters validated the effectiveness of the proposed control algorithm. [ABSTRACT FROM AUTHOR]

To guarantee system stability, proportional capacitor-current-feedback (CCF) active damping control has been widely used in LCL-type grid-connected inverter. However, when digital control is adopted in a grid-side current-controlled inverter, proportional CCF active damping will show negative damping characteristic in frequency band higher than one-sixth of the sampling frequency (fs/6), causing poor robustness in weak grid. In this article, an unstable second-order phase lead compensator is proposed to insert into the CCF path for eliminating the negative effect resulting from the digital control delay. In doing so, the positive damping region is extended to almost Nyquist frequency (fs/2), which achieves high robustness against grid impedance variation. To ensure system stability, the discrete rule is investigated for the proposed compensator and system performance is analyzed when the LCL resonance frequency changes in the range of (0, fs/2) in discrete domain, a particularly simple and practical design is further presented for the proposed compensator and the current regulator. Finally, the effectiveness of the proposed strategy and parameters design is verified by the experimental results. [ABSTRACT FROM AUTHOR]

Grid-connected inverters inevitably operate under uncertain system parameters. These uncertain parameters such as inductance and capacitance are caused by device aging or uncertain grid conditions. And they will affect the stability margin of the inverter and bring an unstable operation risk even with a well initial design. The existing research articles mainly focus on robust stability analysis and control strategies while ignoring the influence of parameter distributions on operational risk. How to evaluate the instability risk of the inverter under multiparameter distribution is still an open question. In this article, the structured singular value (SSV) method is proposed to evaluate the robust stability of the inverter with multiparameter distributions. The distribution characteristics of uncertain parameters are briefly reviewed. The system model of a single-phase grid-connected inverter is established with the parameter distributions of the system key components. The probability of stable system operation can be quantified by the SSV method. Experiment and case study results verify the correctness and effectiveness of the proposed method. [ABSTRACT FROM AUTHOR]

The fast short-circuit fault ride-through capability of the inverter is an important indicator for the reliability of an independent power supply system. Fast short-circuit fault ride-through requires the inverter to output the short-circuit fault current quickly and reliably, which is conducive to shortening the duration of short-circuit faults and realizing fast short-circuit protection. In addition, when the single-phase inverter outputs a short-circuit fault current, the third harmonic in the short-circuit current may cause malfunction of the protection circuit. To effectively achieve the fast short-circuit current limiting and third harmonic suppression of the single-phase inverter, this article proposes a short-circuit fault diagnosis strategy and a current-limiting control strategy, respectively. The former is a short-circuit fault diagnosis strategy, and the latter is a fast current limiting control strategy based on adaptive reference feedforward and third harmonic elimination. The proposed strategies can enable the single-phase inverter to reach a current-limiting steady state within a fundamental period, which can greatly improve the response speed of single-phase inverters and achieve fast short-circuit protection. They can also be applied to three-phase inverters. A 35-kVA/50-Hz high-power single-phase inverter prototype is built to verify the effectiveness of the proposed method. [ABSTRACT FROM AUTHOR]

The frequency-domain passivity theory offers an effective way to assess the stability of inverters in a complex grid. In this article, a unified impedance model, suitable for either inverter current control (ICC) or grid current control (GCC) of LCL-type grid-connected inverters with observer-based capacitor current feedback active damping, is built to facilitate the passivity-based stability assessment and controller parameter design. With the passivity analysis, it is found that when the antiresonant frequency of LCL filter is in certain ranges, i.e., (0.056 $\omega _s$ , 0.20 $\omega _s$) for ICC and (0.046 $\omega _s$ , 0.23 $\omega _s$) for GCC, all frequencies’ passive output admittance of the inverter can be achieved via proposed parameter design guidelines. Due to the utilization of the observer and all frequencies’ passive output admittance property, not only extra current sensors for active damping can be saved, but also the inverter can be connected and stably operated in a grid regardless of the grid impedance. The validity of the theoretical analysis and effectiveness of the proposed method are verified by using experimental results on a laboratory prototype. [ABSTRACT FROM AUTHOR]

Doubly grounded transformerless grid-connected inverters are widely used in the PV application because of no common mode leakage current. A boost converter should be added into the doubly grounded PV inverters under shading condition to have boost function. However, the inverter with boost capability is a two-stage system and all switches are operated at high frequency, which reduce efficiency of the system. In addition, processing power of the boost converter is high. Therefore, a single-stage doubly grounded transformerless PV grid-connected inverter with boost function is proposed. The proposed inverter consists of two boost and one buck converters. A cooperative control method among different periods is proposed to reduce processing power of the boost converter and simplify the two-stage system to a single-stage system in each period. In addition, only one switch is operated at high frequency in each period, so efficiency of the system can be improved. Operating principle of the proposed inverter is illustrated. Control strategy is designed. Filter design guidelines and example are given. Loss calculation of the proposed inverter is given. Simulation and experimental results of the proposed inverter verify the theoretical analysis. Finally, comparison among other inverters and the proposed inverter is provided. [ABSTRACT FROM AUTHOR]

In medium/high power applications, including smart transformers, active power filters, and wind turbines, three-level neutral-point-clamped (NPC) inverters proved to be a reliable solution, providing high efficiency and low harmonic distortion. In practice, several NPCs are parallel connected and operated in interleaved manner to further increase the power handling and reduce the line filters size. However, if such configuration has a common dc-link, high-frequency zero-sequence circulating current (HF-ZSCC) arises among the inverters, increasing power losses of the switching devices and propagating the stress on the dc-capacitors. Moreover, the amplitude of the HF-ZSCC is inversely proportional to the filter inductance size, therefore in real applications, it can reach hundreds of amperes even with relatively low output currents. The research on the HF-ZSCC is mostly concentrated on two-level inverters for low voltage grids and traction applications, where the inductance size is relatively big and the HF-ZSCC does not affect the system efficiency. Differently, NPCs provide higher switching degree of freedom and more sophisticated methods can be applied to reduce the HF-ZSCC. This article investigates a double-reference pulsewidth modulation (DRPWM) as solution for diminishing the HF-ZSCC in paralleled NPCs. The performance of DRPWM method is confirmed by both simulation and experiments, performed on a 1.6-MVA system. [ABSTRACT FROM AUTHOR]